Drilling assemblies have downhole motors and other mechanisms to achieve directional drilling. Referring to FIG. 1A, for example, a drilling assembly 10 connects to a drillstring 14 and has a drill bit 12 rotatably connected to a downhole motor 20. A rig 16 at the surface can rotate the drillstring 14 and the assembly 10, and surface equipment 18 including mud pumps can pump drilling fluid or mud down the drillstring 14 to the downhole motor 20. Operated by the flow of drilling fluid, the downhole motor 20 can also impart rotation to the drill bit 12.
In general, the downhole motor 20 as shown in FIGS. 1A-1B has a housing 22, a power section 24, a transmission section 26, and a bearing section 28. Drilling fluid pumped through the motor 20 actuates the power section 24, which drives a mandrel 29 through the transmission section 26 to rotate the drill bit (12). The bearing section 28 supports the motor's drive mandrel 29.
The environment encountered by the downhole motor 20 is extremely hostile. For example, the motor 20 is continuously exposed to very high temperatures over very long periods of time. Therefore, the bearing section 28 in the motor 20 may occasionally fail, which prevents the free rotation of the drive mandrel 29 relative to the motor housing 22. When this occurs, portions of the motor housing 22 below the power section 24 tend to rotate with the rotational force applied by the power section 24 to the drill bit 12.
Should the bearings in the bearing section 28 cease to operate properly, for example, then the rotational force applied to the drill bit 12 is also applied to the motor housing 22. Eventually, portions of the motor's housing 22 can separate from one another, and portions of the motor 20 can possibly become lost in the well. Typically, the portions of the housing 22 are attached with right hand threads. Therefore, the clockwise rotation of the portions of the housing 22 relative to one another tends to unscrew sections of the housing 22 until they separate.
A separation catch mechanism for preventing separation of a downhole motor from a drillstring has become standard equipment on motors used for directional drilling. One apparatus available in the art is disclosed in U.S. Pat. No. 5,165,492 by Dailey Petroleum Service Corp. Another apparatus is disclosed in U.S. Pat. No. 7,063,175 to Kerstetter.
As an example, FIG. 1B shows one type of apparatus for preventing separation of the downhole motor 20 from the drillstring. The motor 20 has an upper housing member 30 coupled to a lower housing member 40 above the power section 24. A rotor extension 60 is coupled to the end of the rotor 44, which is disposed for rotation in the stator 42 of the power section 24. Should portions of the motor's housing 22 separate during operation, a head 62 on the end of the extension 60 can engage a seat 35 in the upper housing member 30 and can prevent the lower housing member 40 and/or other portions of the motor 20 from separating completely from the upper housing member 30 and drillstring.
Yet another apparatus is used in Weatherford's Hyperline Drilling Motor. An example of this apparatus is illustrated in FIG. 2, which shows only an upper portion of a motor 20. Again, the motor 20 has an upper housing member 30 with a threaded end 34 coupled to a lower housing member 40. An upper threaded end 36 of the upper member 30 can affix to other tubular members of the drillstring (not shown). The lower housing member 40 supports a stator 42 with a rotor 44 disposed for rotation therein. Flow of drilling fluid in the space between the rotor 44 and stator 42 rotates the rotor 44, which in turn rotates a drill bit (not shown) further downhole on the motor 20.
The upper member 30 has an internal passage 32 separated at its upper end from the upper threads 36 by a reduced passage 35. The internal passage 32 at its lower end has internal threads 38 to which a seat 50 threads. A rotor extension 60 threads at one end 64 to the rotor 44, and the other end of the rotor extension 60 has a head 62, which positions within the internal passage 32.
Assembly of the apparatus involves separately affixing the components of the housing members 30 and 40, the seat 50, and the rotor extension 60 so that the rotor extension 60 can be held within the upper housing member 30. Should some lower housing portions of the motor 20 separate from one another, then the head 62 on the distal end of the rotor extension 60 can engage the seat 50 and prevent the lower components of the motor 20 from fully separating from the upper motor components and the drillstring.
The separation catch mechanisms, such as discussed above, have performed adequately for many years. However, new developments in the power sections of motors and in the technology of drill bits have increased drilling rates by increasing the torque on the drill bit. These new developments have also been combined with changes to drilling practices, such as drilling both build and tangent sections of a borehole with a single motor bend setting (i.e., performing rotary drilling with significant bend in the motor). Under these conditions, the bending stiffness in the existing separation catch mechanisms may be imbalanced, which can result in connection fractures. In particular, the mechanism of FIG. 2 requires an excessively stiff pin end 34 due to the required step in the pin's internal dimension to accommodate the seat 50.
The subject matter of the present disclosure is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.